The present invention relates to an optical head used in an optical recording/reproducing apparatus. More particularly, the present invention relates to an optical head with high light utilization efficiency that is provided with a diffractive optical element and a light source emitting beams with a plurality of wavelengths and that can read plural types of information recording media.
Optical heads are important components for reading signals from optical recording media such as optical disks, for example, compact disks (CDs) and digital video disks (DVDs), and optical card memories. Not only signal detection functions, but also control mechanisms, such as focus servos or tracking servos, are necessary for optical heads to read out signals from an optical recording medium.
Generally, an optical head comprises various optical components such as a light source, a photodetector, a condenser lens (an objective lens), a focusing/tracking error signal detection element, a mirror for bending an optical path, a collimator lens, and the like. A laser beam emitted from a light source is focused on an optical disk by an objective lens. The laser beam focused on the optical disk is reflected and then is detected by the photodetector. Thus, a reproduction signal is read out. In addition, the focusing/tracking error signal detection element controls focusing and tracking, thus enabling signals to be read out stably.
When a diffractive optical element is used as an optical element in the optical head instead of a refraction optical element such as a general lens, prism, or the like, the optical head can be reduced in size, in thickness, and in weight.
The diffractive optical element denotes an optical element that functions by utilizing diffraction phenomenon. The diffractive optical element is characterized in that a corrugated structure having depth equal to a wavelength order are formed on its surface periodically or quasi-periodically or its surface is formed so that refractive index or amplitude is distributed periodically or quasi-periodically. It has been known that when a period of the diffractive optical element is sufficiently large compared to a wavelength of light being incident onto the diffractive optical element, the diffraction efficiency can be increased almost to 100% by forming the diffractive optical element so as to have a cross section of a sawtooth shape.
When the period of the diffractive optical element is sufficiently large compared to the wavelength of light being incident onto the diffractive optical element, however, the diffraction efficiency of the diffractive optical element can reach 100% only with respect to a design wavelength. FIG. 14 shows the relationship between a wavelength normalized by a design wavelength and first-order diffraction efficiency of the diffractive optical element. As can be seen from FIG. 14, the diffraction efficiency decreases gradually as the wavelength deviates from the design value. Therefore, when the diffractive optical element is used in an optical head in which a light source emitting beams with a plurality of wavelengths corresponding to plural types of optical disks is mounted, the diffractive optical element has been required to be designed for each wavelength optimally and to be positioned only in the optical path of a beam with the intended wavelength to increase the light utilization efficiency.
In one or more embodiments, the present invention aims to solve the aforementioned problem in the conventional technique and to provide an optical head with high light utilization efficiency that includes a diffractive optical element and a light source emitting beams with a plurality of wavelengths for reading plural types of information recording media.
A first configuration of an optical head according to the present invention includes: at least one light source for emitting a beam with a first wavelength and a beam with a second wavelength that is approximately twice as long as the first wavelength; a photodetector; and at least one diffractive optical element provided in optical paths of the beams with the first and the second wavelengths. In the optical head, outgoing light from the diffractive optical element is substantially a second-order diffraction light with respect to the beam with the first wavelength and is substantially a first-order diffraction light with respect to the beam with the second wavelength.
According to the first configuration of the optical head, for example, even when the diffractive optical element is positioned in optical paths of beams with two wavelengths, high diffraction efficiency can be obtained with respect to both the wavelengths, thus obtaining an optical head with excellent optical characteristics.
In the optical head according to the above-mentioned first configuration, it is preferable that the diffractive optical element has a cross section substantially of a sawtooth shape, and with respect to the first wavelength xcex1, the second wavelength xcex2, and a refractive index n of a material of the diffractive optical element, the groove depth in the sawtooth shape is substantially in a range between 2xcex1/(nxe2x88x921) and xcex2/(nxe2x88x921) in the case of a transmission element, is substantially in a range between xcex1/n and xcex2/2n in the case of a reflection element onto which beams are incident from a substrate side, and is substantially in a range between xcex1 and xcex2/2 in the Do case of a reflection element onto which beams are incident from an air side.
According to this preferable configuration, the highest diffraction efficiency of the diffractive optical element can be obtained, for example, with respect to the beams with the first and the second wavelengths.
In the optical head according to the above-mentioned first configuration, it is preferable that the diffractive optical element is an objective lens for focusing beams on an information recording medium.
According to this preferable configuration, for example, the objective lens can be reduced in thickness and in weight.
In the optical head according to the above-mentioned first configuration, it is preferable that the diffractive optical element is a collimator lens for collimating beams emitted from the light source substantially into parallel beams.
According to this preferable configuration, for example, the collimator lens can be reduced in thickness and in weight.
In the optical head according to the above-mentioned first configuration, it is preferable that the diffractive optical element is a focusing/tracking error signal detection element.
According to this preferable configuration, for example, the focusing/tracking error signal detection element can be reduced in thickness and in weight.
In the optical head according to the above-mentioned first configuration, it is preferable that with respect to the first wavelength xcex1, a minimum period xcex9min of the diffractive optical element satisfies a relationship of xcex9minxe2x89xa710xcex1.
According to this preferable configuration, for example, the diffractive optical element with a high diffraction efficiency of at least 80% with respect to the beam with the first wavelength can be obtained.
In the optical head according to the above-mentioned first configuration, it is preferable that with respect to the first wavelength xcex1, a minimum period xcex9min of the diffractive optical element satisfies a relationship of xcex9minxe2x89xa722xcex1.
According to this preferable configuration, for example, the diffractive optical element with a higher diffraction efficiency of at least 90% with respect to the beam with the first wavelength can be obtained.
In the optical head according to the above-mentioned first configuration, it is preferable that a refraction optical element having optical surfaces onto which beams emitted from the light source are incident obliquely is provided in optical paths of the beams with the first and the second wavelengths, and a diffraction angle of light diffracted by the diffractive optical element due to variation in wavelength of the beams emitted from the light source and a refraction angle of light refracted by the refraction optical element are changed in directions that enable them to be canceled out with each other.
According to this preferable configuration, for example, a thin optical head can be obtained and in addition, when a semiconductor laser beam is used as a beam emitted from the light source, an excellent focusing spot can be obtained on an optical disk surface even when a central wavelength of the beam emitted from the light source is varied according to the broadening of a wavelength range of about several nm due to a high-frequency module or self-oscillation or the change in environmental temperature.
In the above-mentioned preferable configuration, it is further preferable that the diffractive optical element is a grating with a uniform period.
According to this further preferable configuration, for example, positioning and manufacture of the diffractive optical element can be facilitated.
In the above-mentioned preferable configuration, it is further preferable that the diffractive optical element is positioned in a converging light optical path or a diverging light optical path with a numerical aperture of 0.39 or less, and the diffractive optical element has a uniform period.
According to this further preferable configuration, for example, positioning and manufacture of the diffractive optical element can be facilitated.
In the above-mentioned preferable configuration, it is further preferable that the refraction optical element is a prism with three optical surfaces, and when in the three optical surfaces, a surface on a side of an information recording medium is a first surface, a surface on a side of the light source is a second surface, and a surface other than those is a third surface, the refraction optical element is designed so that the beams emitted from the light source pass through the second surface, are reflected by the first and the third surfaces sequentially, and then pass through the first surface, and a bottom part of an objective lens is located lower than a highest position of the beams entering the second surface that have been emitted from the light source.
According to this further preferable configuration, for example, the optical head can be reduced in thickness.
In the above-mentioned further preferable configuration, it is still preferable that a glass material of the prism has an Abbe number of at least 64.
According to this still preferable configuration, for example, manufacture of the diffractive optical element can be facilitated due to the enlargement in period of the diffractive optical element and high diffraction efficiency can be obtained and the influence of variation in wavelength can be cancelled out in a wide wavelength range, thus obtaining an excellent focusing spot on an optical disk surface.
In the above-mentioned first configuration, it is preferable that the light source is an SHG light source that emits beams with two wavelengths.
According to this preferable configuration, for example, a light source for emitting a beam with a first wavelength and a beam with a second wavelength that is approximately twice as long as the first wavelength can be obtained.
In the above-mentioned first configuration, it is preferable that a transparent substrate in which the beams with the first and the second wavelengths propagate in a zigzag manner is provided and the diffractive optical element is positioned on the transparent substrate.
According to this preferable configuration, for example, a thin optical head with a stable structure can be obtained.
In the above-mentioned first configuration, it is preferable that the first wavelength xcex1 satisfies a relationship of 0.35 xcexcmxe2x89xa6xcex1xe2x89xa60.44 xcexcm or the second wavelength xcex2 satisfies a relationship of 0.76 xcexcmxe2x89xa6xcex2 0.88 xcexcm.
According to this preferable configuration, for example, a small focusing spot can be obtained, and high-density disks, for example, with at least 10 GByte capacity or optical disks such as CDs and CD-Rs can be read.
In the above-mentioned first configuration, it is preferable that the first wavelength xcex1 satisfies substantially a relationship of 0.35 xcexcmxe2x89xa6xcex1xe2x89xa60.44 xcexcm, and the diffractive optical element is a chromatic-aberration compensation element for compensating chromatic aberration caused by an objective lens for focusing beams on an information recording medium.
According to this preferable configuration, for example, an excellent focusing spot can be obtained on an optical disk surface by compensating great chromatic aberration caused by the objective lens when a semiconductor laser beam with a wavelength in a range of 0.35 xcexcmxe2x89xa6xcex1xe2x89xa60.44 xcexcm in which great chromatic dispersion is caused by the glass material of a lens is used as a beam emitted from the light source, even when a central wavelength of the beam emitted from the light source is varied according to the broadening of a wavelength range of about several nm due to a high-frequency module or self-oscillation or the change in environmental temperature.
A second configuration of an optical head according to the present invention includes: at least one light source for emitting a beam with a first wavelength and a beam with a second wavelength that is approximately twice as long as the first wavelength; a photodetector; an objective lens for focusing beams on an information recording medium; and a diffractive optical element provided in optical paths of the beams with the first and the second wavelengths. In the optical head, the diffractive optical element is a chromatic-aberration compensation element that compensates chromatic aberration caused by the objective lens and that has difference in level of a step-like shape or grooves substantially of a sawtooth shape, and with respect to the first wavelength xcex1, the second wavelength xcex2, and a refractive index n of a material of the chromatic-aberration compensation element, the difference in level or the groove depth is in a range substantially between 2xcex1/(nxe2x88x921) and xcex2/(nxe2x88x921).
According to the above-mentioned second configuration, for example, a chromatic-aberration compensation element with excellent light utilization efficiency for the beams with the first and the second wavelengths can be obtained.
In the above-mentioned second configuration, it is preferable that the first wavelength xcex1 satisfies a relationship of 0.35 xcexcmxe2x89xa6xcex1xe2x89xa60.44 xcexcm or the second wavelength xcex2 satisfies a relationship of 0.76 xcexcmxe2x89xa6xcex2xe2x89xa60.88 xcexcm.
In the above-mentioned second configuration, it is preferable that the chromatic-aberration compensation element is formed on the objective lens.
According to the above-mentioned preferable configuration, for example, the chromatic-aberration compensation element and the objective lens can be handled as one component, thus reducing the size and cost of the optical head.
In the above-mentioned second configuration, it is preferable that the chromatic-aberration compensation element and the objective lens are driven together by an actuator.
According to the above-mentioned preferable configuration, for example, an optical axis of the chromatic-aberration compensation element and that of the objective lens are not shifted from each other, thus obtaining excellent optical characteristics.
In the above-mentioned second configuration, it is preferable that the chromatic-aberration compensation element is a convex diffractive lens and forms converging light in cooperation with the objective lens.
According to the above-mentioned preferable configuration, for example, a numerical aperture of the objective lens itself can be reduced, thus facilitating the manufacture of the objective lens.
A third configuration of an optical head according to the present invention includes: at lest one light source for emitting a beam with a first wavelength, a beam with a second wavelength that is approximately twice as long as the first wavelength, and a beam with a third wavelength that is approximately 1.5 times as long as the first wavelength; a photodetector; and at least one diffractive optical element provided in optical paths of the beams with the first, the second, and the third wavelengths. In the optical head, outgoing light from the diffractive optical element is substantially a fourth-order diffraction light with respect to the beam with the first wavelength, is substantially a second-order diffraction light with respect to the beam with the second wavelength, and is substantially a third-order diffraction light with respect to the beam with the third wavelength.
According to the above-mentioned third configuration, for example, high diffraction efficiency can be obtained particularly for the beams with the first and the second wavelengths out of the beams with the three wavelengths even when the diffractive optical element is positioned in the optical path common to the beams with the first to the third wavelengths, thus obtaining an optical head with excellent optical characteristics.
In the above-mentioned third configuration, it is preferable that the diffractive optical element has a cross section substantially of a sawtooth shape, and with respect to the first wavelength xcex1, the second wavelength xcex2, the third wavelength xcex3, and a refractive index n of a material of the diffractive optical element, the groove depth in the sawtooth shape is substantially in a range between a minimum and a maximum among 4xcex1/(nxe2x88x921), 2xcex2/(nxe2x88x921), and 3xcex3/(nxe2x88x921) in the case of a transmission element, is substantially in a range between a minimum and a maximum among 2xcex1/n, xcex2/n, and 3xcex3/2n in the case of a reflection element onto which beams are incident from a substrate side, and is substantially in a range between a minimum and a maximum among 2xcex1, xcex2, and 3xcex3/2 in the case of a reflection element onto which beams are incident from an air side.
According to the above-mentioned preferable configuration, for example, a diffractive optical element with highest diffraction efficiency can be obtained particularly for the beams with the first and the second wavelengths out of the beams with the first to the third wavelengths.
In the above-mentioned third configuration, it is preferable that with respect to the first wavelength xcex1, a minimum period xcex9min of the diffractive optical element satisfies a relationship of xcex9minxe2x89xa722xcex1.
According to the above-mentioned preferable configuration, for example, a diffractive optical element with a high diffraction efficiency of at least 80% with respect to the beam with the first wavelength can be obtained.
In the above-mentioned third configuration, it is preferable that the first wavelength xcex1 satisfies a relationship of 0.35 xcexcmxe2x89xa6xcex1xe2x89xa60.44 xcexcm, the second wavelength xcex2 satisfies a relationship of 0.76 xcexcmxe2x89xa6xcex2xe2x89xa60.88 xcexcm, or the third wavelength xcex3 satisfies a relationship of 0.57 xcexcmxe2x89xa6xcex3xe2x89xa60.68 xcexcm.
According to the above-mentioned preferable configuration, for example, a small focusing spot can be obtained and thus high-density optical disks with, for instance, at least 10 Gbyte capacity, optical disks such as CDs and CD-Rs, or optical disks such as DVDs and DVD-Rs including a two-layer structure can be read.
In the above-mentioned third configuration, it is preferable that the first wavelength xcex1 satisfies substantially a relationship of 0.35 xcexcmxe2x89xa6xcex1xe2x89xa60.44 xcexcm, and the diffractive optical element is a chromatic-aberration compensation element for compensating chromatic aberration caused by an objective lens for focusing beams on an information recording medium.
A fourth configuration of an optical head according to the present invention includes: at least one light source for emitting a beam with a first wavelength, a beam with a second wavelength that is approximately twice as long as the first wavelength, and a beam with a third wavelength that is approximately 1.5 times as long as the first wavelength; a photodetector; an objective lens for focusing beams on an information recording medium; and at least one diffractive optical element provided in optical paths of the beams with the first, the second, and the third wavelengths. In the optical head, the diffractive optical element is a chromatic-aberration compensation element that compensates chromatic aberration caused by the objective lens and has difference in level of a step-like shape or grooves substantially of a sawtooth shape, and with respect to the first wavelength xcex1, the second wavelength xcex2, the third wavelength xcex3, and a refractive index n of a material of the diffractive optical element, the difference in level or the groove depth is in a range between a minimum and a maximum among 4xcex1/(nxe2x88x921), 2xcex2/(nxe2x88x921), and 3xcex3(nxe2x88x921).
According to the above-mentioned fourth configuration, for example, a chromatic-aberration compensation element with excellent light utilization efficiency particularly for the beams with the first and the second wavelengths out of the beams with the first to the third wavelengths can be obtained.
In the above-mentioned fourth configuration, it is preferable that the first wavelength xcex1 satisfies a relationship of 0.35 xcexcmxe2x89xa6xcex1xe2x89xa60.44 xcexcm, the second wavelength xcex2 satisfies a relationship of 0.76 xcexcmxe2x89xa6xcex2xe2x89xa60.88 xcexcm, or the third wavelength xcex3 satisfies a relationship of 0.57 xcexcmxe2x89xa6xcex3xe2x89xa60.68 xcexcm.
In the above-mentioned fourth configuration, it is preferable that the chromatic-aberration compensation element is formed on the objective lens.
In the above-mentioned fourth configuration, it is preferable that the chromatic-aberration compensation element and the objective lens are driven together by an actuator.
In the above-mentioned fourth configuration, it is preferable that the chromatic-aberration compensation element is a convex diffractive lens and forms converging light in cooperation with the objective lens.
A fifth configuration of an optical head according to the present invention includes: at least one light source for emitting a beam with a first wavelength and a beam with a second wavelength that is approximately 1.5 times as long as the first wavelength; a photodetector; and at least one diffractive optical element provided in optical paths of the beams with the first and the second wavelengths. In the optical head, outgoing light from the diffractive optical element is substantially a third-order diffraction light with respect to the beam with the first wavelength and is substantially a second-order diffraction light with respect to the beam with the second wavelength.
According to the above-mentioned fifth configuration, for example, high diffraction efficiency can be obtained for both the beams with the first and the second wavelengths even when the diffractive optical element is positioned in the optical path common to the beams with the first and the second wavelengths, thus obtaining an optical head with excellent optical characteristics.
In the above-mentioned fifth configuration, it is preferable that the diffractive optical element has a cross section substantially of a sawtooth shape, and with respect to the first wavelength xcex1, the second wavelength xcex2, and a refractive index n of a material of the diffractive optical element, the groove depth in the sawtooth shape is substantially in a range between 3xcex1/(nxe2x88x921) and 2xcex2/(nxe2x88x921) in the case of a transmission element, is substantially in a range between 3xcex1/2n and xcex2/n in the case of a reflection element onto which beams are incident from a substrate side, and is substantially in a range between 3xcex1/2 and xcex2 in the case of a reflection element onto which beams are incident from an air side.
According to the above-mentioned preferable configuration, for example, a diffractive optical element with highest diffraction efficiency for the beams with the first and the second wavelengths can be obtained.
In the above-mentioned fifth configuration, it is preferable that with respect to the first wavelength xcex1, a minimum period xcexmin of the diffractive optical element satisfies a relationship of xcex9minxe2x89xa716xcex1.
According to the above-mentioned preferable configuration, for example, a diffractive optical element with a high diffraction efficiency of at least 80% with respect to the beam with the first wavelength can be obtained.
In the above-mentioned fifth configuration, it is preferable that the first wavelength xcex1 satisfies a relationship of 0.35 xcexcmxe2x89xa6xcex1xe2x89xa60.44 xcexcm or the second wavelength xcex2 satisfies a relationship of 0.57 xcexcmxe2x89xa6xcex2xe2x89xa60.68 xcexcm.
In the above-mentioned fifth configuration, it is preferable that the first wavelength xcex1 satisfies substantially a relationship of 0.35 xcexcmxe2x89xa6xcex1xe2x89xa60.44 xcexcm, and the diffractive optical element is a chromatic-aberration compensation element for compensating chromatic aberration caused by an objective lens for focusing beams on an information recording medium.
A sixth configuration of an optical head according to the present invention includes: at least one light source for emitting a beam with a first wavelength and a beam with a second wavelength that is approximately 1.5 times as long as the first wavelength; a photodetector; an objective lens for focusing beams on an information recording medium; and a diffractive optical element provided in optical paths of the beams with the first and the second wavelengths. In the optical head, the diffractive optical element is a chromatic-aberration compensation element that compensates chromatic aberration caused by the objective lens and has difference in level of a step-like shape or grooves substantially of a sawtooth shape, and with respect to the first wavelength xcex1, the second wavelength xcex2, and a refractive index n of a material of the chromatic-aberration compensation element, the difference in level or the groove depth is substantially in a range between 3xcex1/(nxe2x88x921) and 2xcex2/(nxe2x88x921).
According to the above-mentioned sixth configuration, for example, a chromatic-aberration compensation element with excellent light utilization efficiency for the beams with the first and the second wavelengths can be obtained.
In the above-mentioned sixth configuration, it is preferable that the first wavelength xcex1 satisfies a relationship of 0.35 xcexcmxe2x89xa6xcex1xe2x89xa60.44 xcexcm or the second wavelength xcex2 satisfies a relationship of 0.57 xcexcmxe2x89xa6xcex2xe2x89xa60.68 xcexcm.
According to the above-mentioned preferable configuration, for example, a small focusing spot can be obtained and thus high-density optical disks with, for example, at least 10 GByte capacity or optical disks such as DVDs and DVD-Rs including a two-layer structure can be read.
In the above-mentioned sixth configuration, it is preferable that the chromatic-aberration compensation element is formed on the objective lens.
In the above-mentioned sixth configuration, it is preferable that the chromatic-aberration compensation element and the objective lens are driven together by an actuator.
In the above-mentioned sixth configuration, it is preferable that the chromatic-aberration compensation element is a convex diffractive lens and forms converging light in cooperation with the objective lens.
A seventh configuration of an optical head according to the present invention includes: at least one light source for emitting a beam with a first wavelength, a beam with a second wavelength that is approximately twice as long as the first wavelength, and a beam with a third wavelength that is approximately 1.5 times as long as the first wavelength; a photodetector; and at least one diffractive optical element provided in optical paths of the beams with the first, the second, and the third wavelengths. In the optical head, outgoing light from the diffractive optical element is substantially a sixth-order diffraction light with respect to the beam with the first wavelength, is substantially a third-order diffraction light with respect to the beam with the second wavelength, and is substantially a fourth-order diffraction light with respect to the beam with the third wavelength.
According to the above-mentioned seventh configuration, for example, high diffraction efficiency can be obtained for the beams with the three wavelengths even when the diffractive optical element is positioned in the optical path common to the beams with the three wavelengths, thus obtaining an optical head with excellent optical characteristics.
In the above-mentioned seventh configuration, it is preferable that the diffractive optical element has a cross section substantially of a sawtooth shape, and with respect to the first wavelength xcex1, the second wavelength xcex2, the third wavelength xcex3, and a refractive index n of a material of the diffractive optical element, the groove depth in the sawtooth shape is substantially in a range between a minimum and a maximum among 6xcex1/(nxe2x88x921), 3xcex2/(nxe2x88x921), and 4xcex3/(nxe2x88x921) in the case of a transmission element, is substantially in a range between a minimum and a maximum among 3xcex1/n, 3xcex2/2n, and 2xcex3/n in the case of a reflection element onto which beams are incident from a substrate side, and is substantially in a range between a minimum and a maximum among 3xcex1, 3xcex2/2, and 2xcex3 in the case of a reflection element onto which beams are incident from an air side.
According to the above-mentioned preferable configuration, for example, a diffractive optical element with highest diffraction efficiency for the beams with the first to the third wavelengths can be obtained.
An eighth configuration of an optical head according to the present invention includes: at least one light source for emitting a beam with a first wavelength, a beam with a second wavelength that is approximately twice as long as the first wavelength, and a beam with a third wavelength that is approximately 1.5 times as long as the first wavelength; a photodetector; an objective lens for focusing beams on an information recording medium; and at least one diffractive optical element provided in optical paths of the beams with the first, the second, and the third wavelengths. In the optical head, the diffractive optical element is a chromatic-aberration compensation element that compensates chromatic aberration caused by the objective lens and that has difference in level of a step-like shape or grooves substantially of a sawtooth shape, and with respect to the first wavelength xcex1, the second wavelength xcex2, the third wavelength xcex3, and a refractive index n of a material of the diffractive optical element, the difference in level or the groove depth is substantially in a range between a minimum and a maximum among 6xcex1/(nxe2x88x921), 3xcex2/(nxe2x88x921), and 4xcex3/(nxe2x88x921).
According to the above-mentioned eighth configuration, for example, a chromatic-aberration compensation element with excellent optical characteristics for the beams with the first to the third wavelengths can be obtained.
In the above-mentioned eighth configuration, it is preferable that the chromatic-aberration compensation element is formed on the objective lens.
In the above-mentioned eighth configuration, it is preferable that the chromatic-aberration compensation element and the objective lens are driven together by an actuator.
In the above-mentioned eighth configuration, it is preferable that the chromatic-aberration compensation element is a convex diffractive lens and forms converging light in cooperation with the objective lens.
A ninth configuration of an optical head according to the present invention includes: a light source emitting a beam with a wavelength xcex that satisfies substantially a relationship of 0.35 xcexcmxe2x89xa6xcexxe2x89xa60.44 xcexcm; a photodetector; and at least one diffractive optical element provided in an optical path of the beam emitted from the light source. In the optical head, the diffractive optical element is a chromatic-aberration compensation element for compensating chromatic aberration caused by an objective lens for focusing beams on an information recording medium.
In the above-mentioned ninth configuration, it is preferable that the chromatic-aberration compensation element is formed on the objective lens.
In the above-mentioned ninth configuration, it is preferable that the chromatic-aberration compensation element and the objective lens are driven together by an actuator.
In the above-mentioned ninth configuration, it is preferable that the chromatic-aberration compensation element is a convex diffractive lens and forms converging light in cooperation with the objective lens.